A PLANARIAN WORM 63 



seen. The mouth is a circular opening near the center of the ven- 

 tral surface ; it leads into the pharynx, a cylindrical organ with 

 thick, muscular walls, which can be thrust out of the mouth as a 

 proboscis. At the base of the pharynx the intestine divides into 

 three trunks, one of which passes forward and the other two back- 

 ward to the extremities of the animal's body. Each of these 

 trunks gives off lateral branches which are themselves often 

 branched. There is no anus. 



Exercise 2. Draw an outline of the animal and place in it the digestive 

 system in detail. 



The Reproductive System. Planarians are hermaphroditic ; 

 the sexual organs are complicated in structure and arrangement 

 and difficult to observe in a Kve specimen. Near the lateral edges 

 of the body will be seen, among the ends of the lateral intestinal 

 branches, two sets of lobed organs. Of these the larger are the 

 yolk glands, which connect with the oviducts ; the smaller and less 

 apparent ones are the rounded testes. Just back of the mouth is 

 the uterus, which is often to be recognized by the spherical eggs 

 it may contain ; it passes back to a sac called the genital cloaca. 

 The ovaries are a pair of spherical bodies in the anterior part of 

 the body, and from them a pair of oviducts extends to the hinder 

 part of the body, receiving the lateral yolk glands on their course. 

 Leading from the testes are the vasa efferentia, very delicate 

 tubes, which pass to the conspicuous vasa deferentia. There is a 

 pair of the latter organs, one on each side of the mouth and phar- 

 ynx ; they extend to the hinder part of the animal, where they 

 unite to form the muscular cirrus, which opens into the genital 

 cloaca. The two oviducts also fuse at their hinder ends, and the 

 median duct thus formed opens into the genital cloaca. This 

 structure, which thus receives all the ducts from the genital glands, 

 communicates with the outside through the genital pore, a median, 

 ventral opening in the hinder part of the body. 



Exercise 3. Draw a large outline of the animal and place in it as 

 much of the reproductive system as you have observed. 



The Nervous System. Beneath the eye spots will be seen the 

 opaque brain, a large nervous mass consisting of a pair of minor 



